The success of hematopoietic stem progenitor cell (HSPC) genetic therapy for HIV-1 is dependent in part upon achieving sufficient levels of marking with therapeutic gene modification to protect the immune system from HIV-1 disease. I propose here to test the hypothesis that a certain threshold level of gene marking is required to effectively replace the HIV-damaged immune system with one that is resistant to HIV-1. Below this threshold, the immune system would not be protected; above the threshold, the immune system would be protected and effectively prevent AIDS. All past and current HSPC clinical trials - which are designed to achieve as high a level of gene marking as is safely possible, anticipating that HIV-1 will ultimately select for the HIV-protected cells - are moving forward without an understanding of this requirement. Herein, I propose to test the hypothesis using mixed bone marrow transplant murine models. I will use a sensitive and high- throughput clonal quantification assay I developed and mathematical modeling to investigate HSPC repopulation and T-cell dynamics above and below this threshold in the presence and absence of selection pressure. Dr. Irvin Chen, Director of the UCLA AIDS Institute and my primary mentor, and his research group were the first to develop lentivirus vectors for HIV-1 gene therapy using short hairpin RNAi against the HIV-1 co-receptor, C-C chemokine receptor type 5 (CCR5), and have demonstrated long-term and stable reduction of CCR5 in both non-human primates and humanized mice. As we prepare to introduce this shRNA into HSPC gene therapy for humans, I will use CCR5 knockdown as the population of T-cells protected from HIV-1 infection and analyze their repopulation kinetics. Knowledge gained from these experiments will help us to determine optimal conditions for efficient and safe selection of engineered cells and to create a mathematical model that can guide further experiments and preclinical trials of engineered HSPC transplantation. I am in an excellent position to pursue this important research thanks to the superior mentoring of Dr. Chen and Dr. Kenneth Lange of Biomathematics; my research and educational backgrounds in both engineering and basic biology; and UCLA's rich research environment. My long-term career goal is to become an independent and specialized translational research scientist in the stem cell gene therapy field. My current research goal is t help introduce the practice of quantitative assessment to better understand and improve treatment conditions and diagnostic/prognostic evaluation in both clinical and preclinical studies of HIV diseases. During the mentored phase of the K99/R00 award, I plan to expand my research expertise in both stem cell gene therapy and mathematical modeling. The research experience I will gain from this training opportunity, along with the financial support granted in the independent phase, will significantly help me to pursue my long-term career and research goals.